This invention generally concerns robotic systems, and is specifically concerned with an improved robotic system for servicing the heat exchanger tubes of a nuclear steam generator that includes a robotic arm capable of providing smoother and more accurate delivery of heavier payloads than prior art systems, and a single consolidated control system capable of simultaneously operating both the arm and a computer controlled end effector.
Because of the radiation hazard present within the steam generators used in a nuclear-powered utility, the heat exchanger tubes of such steam generators must be, for the most part, remotely serviced to avoid exposing maintenance personnel to potentially harmful radiation. Consequently, a number of robotic systems have been developed for remotely preforming repair and maintenance operations on these heat exchanger tubes These robotic systems generally include some sort of robotic delivery arm in combination with any one of a number of specialized tools designed to be carried by the robotic arm, which are known in the art as "end effectors". The type of robotic arms that are presently available on the market fall into two basic categories, which shall be referred to in this application as full movement arms, and limited movement arms. Full movement arms are capable of maintaining an end effector at a desired orientation while moving it along a trajectory having components in all three spatial axes. By contrast, limited movement arms are capable of moving an end effector only along a selected two dimensional trajectory, and cannot maintain the end effector at a desired orientation along this trajectory. The mechanical action of limited movement robotic arms often resembles the operation of a compass used to draw circles, i.e., one end of the arm is pivotally mounted at a point on the flat tubesheet within the channel head of the nuclear steam generator, while the middle portion of the arm is telescopically extendible or retractable. Such arms are capable of sweeping their distal, tool-holding ends across any one of a number or arcs of greater or lesser radii which intersect with desired delivery points on the tubesheet. An example of such a limited movement robotic arm is the Model SM-10 arm manufactured by Zetech located in Isaquah, Washington.
Full movement arms differ from the relatively simplistic structure of the previously described arms in that they include six different segments which are articulated at six different motor-driven joints, which in turn allows movement around six different axes. The more complex structure of such robotic arms allows them to use three of their axes of movement to hold an end effector at a desired orientation, and the other three axes to move the end effector across an infinite number of trajectories in three dimensions while maintaining the end effector at the desired orientation. Such abilities are highly advantageous in situations where it is essential to maintain the end effector at a constant orientation during a servicing operation, as is often the case with a weld head being moved around the location of a desired weld seam. While limited movement arms are often necessarily dedicated to the delivery and manipulation of a single end effector, such as for example a tube inspection probe, full movement arms have the ability in theory to couple onto and de-couple from a variety of end effectors. One of the most advanced designs of such full movement robotic arms is the ROSA (remotely operated service arm) developed by the Westinghouse Electric Corporation located in Pittsburgh, Pennsylvania.
Unfortunately, neither of these two types of robotic arm is without drawbacks. While limited movement robotic arms are relatively simple and inexpensive to construct and to install in nuclear steam generators, the fact that they are typically dedicated to a single end effector necessitates the installation and removal of a number of such arms to complete inspection and servicing operations on the heat exchanger tubes within the channel head of the steam generator. This is a significant shortcoming as every such installation procedure is not only laborious and time-consuming, but also results in the exposure of the operating personnel to potentially harmful radiation. This last drawback is of growing importance, as the NRC (Nuclear Regulatory Commission) has recently placed greater limitations upon the amount of radiation exposure that such operating personnel may absorb. Moreover, the fact that such limited movement robotic arms cannot maintain an end effector at a desired orientation while simultaneously moving it across a chosen trajectory across the tubesheet renders them useless for end effectors that require a constant orientation, such as weld heads. Of course, full movement robotic arms such as the aforementioned ROSA are not limited in these ways. However, prior art full movement robotic arms such as the ROSA also have limitations that offer room for improvement. Specifically, the applicants have noticed that the vertically oriented "elbow" of the ROSA disadvantageously limits the number and length of the possible trajectories that the distal end of the arm may make without mechanically interfering with the bowl-like wall of the channel head, or the divider plate within the channel head, or the cables which vertically drape down from end effectors such as the eddy current probes used to inspect and determine the condition of the interior walls of the heat exchanger tubes. The applicants have also observed that the prior art ROSA is configured so that a large portion of the arm is cantileverly supported from its vertically oriented elbow, which in turn applies a significant amount of life-reducing extraneous torque to the electric motor driving the joint, and reduces its payload carrying ability. Further, the applicants have observed that the motion of the distal end of such robotic arms is not smooth enough to conduct certain welding operations. This lack of smoothness has been found to arise from the fact that the resolvers that are coupled to the outputs of the drive trains which rotate the articulated joints generate a feedback signal which is unfortunately characterized by a certain degree of "noise" which becomes superimposed over the power input to the electric motor as the result of the feedback loop. Several proposals have been made to eliminate the resulting undesirable "jerkiness" of the movement of such arms by processing the feedback signal to smooth the noise out before it enters the central processing unit which modulates the power into the electric motor that turns the joint. However, none of these proposals has met with the desired success. Because the feedback signal must be conducted to the CPU in real time, any processing which results in signal delays is unacceptable. Accordingly, it has been proposed to process the feedback signal by anticipating the shape of the feedback signal, and eliminating it without any real time delays. However, the shape of the curve of such noise is highly complex, and hence is essentially anticipatable, due to the presence of complex "wind-up" torques present in the shafts of the resolvers at the start of any desired arm trajectory. Still other limitations that interfere with the usefulness of known, full movement arms is the fact that the control systems for the arms and the end effectors delivered by these arms are completely separate systems that require their own cable penetrations into the containment area of the facility. The resulting large number of cables results in long set-up times, and further requires the decontamination or discarding of long lengths of expensive cable after each servicing operation.
Clearly there is a need for a full movement robotic arm which is not limited by the drawbacks associated with prior art arms. Specifically, such an arm should be capable of reaching most, if not all of the areas of the tubesheet within the channel head of the steam generator without mechanically interfering with either the bowl-shaped walls of the channel head, the divider plate, or with cables handing down from the various end effectors used in conjunction with the arm. Moreover, the arm should be capable of handling large payloads without the application of excessive torques on the electric motors driving the various joints of the arm, and should further be capable of accurately and reliably delivering an end effector to a location on the tubesheet at any desired orientation. Ideally, the motion of the robotic arm should be sufficiently smooth to conduct very fine welding operations, or any other precision operation associated with the servicing of the steam generator. Finally, the control system associated with such an arm should be easy to setup in a very short period of time within the nuclear facility, and should minimize the number of decontamination tasks after the maintenance operation has been performed.